The present invention relates to a revolver type developing device for a copier, facsimile apparatus, printer or similar image forming apparatus. More particularly, the present invention is concerned with a revolver type developing device having a plurality of developing units arranged around a rotary shaft and each storing a powdery developer of particular color. In this type of developing device, one of the developing units is brought to a developing device at a time so as to image develop a latent image electrostatically formed on an image carrier.
A full-color image forming apparatus, e.g., copier is conventional which scans an image carrier with a color-separated optical image to form a latent image, develops the latent image with a toner complementary in color to the optical image, and repeats such a procedure while transferring the resulting toner images from the image carrier to a single paper or similar recording medium one above the other. Also known in the art is a multicolor image forming apparatus which exposes an image carrier to optical images expected to be reproduced in different colors independently of each other, develops each of the resulting latent images in a particular color to form a corresponding toner image, and transfers such toner images to a single paper one above the other. In any case, this kind of image forming apparatus needs a plurality of developing units for effecting development in a plurality of colors. The problem with such an apparatus is that arranging the developing units side by side in the vicinity of the image carrier makes the developing device and, therefore, the entire image forming apparatus bulky.
In light of the above, there has been proposed a revolver type developing device having a rotatable body adjoining an image carrier. The rotatable body accommodates a plurality of developing units therein which are each located at a particular position. The rotatable body is rotated to sequentially bring the developing units to a developing position facing the image carrier, so that each developing unit develops a respective latent image with a toner of particular color. This type of developing device is taught in, for example, Japanese Utility Model Laid-Open Publication Nos. 51-108639 and 52-110442. Such a device, however, brings about another problem that toners are apt to leak or fly about off from the developing units located at positions other than the developing position, since each developing unit has an opening which will face the image carrier when moved to the developing position. The toners leaked or scattered around contaminate optics, among others, and enter the other developing units to cause color mixture to occur.
To eliminate the problem stated above, the revolver type developing device may be enclosed by a casing or similar screening member having an opening, as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 58-172660, 60-162271 and 60-238872. The screening member successfully prevents the toners from leaking or flying about out of the developing units.
However, the casing scheme taught in, for example, the above-mentioned Laid-Open Publication No. 58-172660 has many issues yet to be solved, as follows.
(1) The casing surrounds the entire revolver and is supported around a shaft which defines the axis of rotation of the revolver. Hence, the casing cannot be reduced in size beyond a certain limit and is complicated and expensive in configuration.
(2) Since the entire revolver is covered with the casing, it is difficult to position the revolver accurately relative to the apparatus body. In addition, the construction of a drive transmission mechanism from the apparatus body to the revolver is limited.
(3) The casing enclosing the entire revolver makes it difficult to build the revolver in the casing.
(4) A current trend in the imaging art is toward a facsimile, printer or similar image forming apparatus having an implementation for user-oriented replacement. In this respect, the revolver with the casing obstructs the replacement of the developing units by the user, safety handing of the developing units, and easy handling of fresh developing units. Particularly, since gears and other drive members built in the revolver are at least partly positioned outside of the casing, they are apt to hurt the operator in the event of, for example, replacement of the developing units.
(5) The revolver is free to rotate relative to the casing. Assume that the revolver is dismounted from the apparatus body, and the positioning of the revolver relative to the casing and the support for the revolver are cancelled. Then, the revolver rotates relative to the casing such that the center of gravity thereof is brought to the lowest position. Specifically, the revolver swings like a pendulum and then stops moving. This kind of behavior is conspicuous with a revolver in use or used since the amount of remaining toner depends on the developing unit. Assume that the revolver behaves in such a manner when the user user or serviceman removes the revolver from the apparatus body from the replacement of the developing units or similar purpose. Then, the center of gravity of the entire revolver, including the casing, is displaced. As a result, the person handling the revolver is apt to lose his balance and drop the revolver or to hit it against surrounding devices or equipment. Also, it is likely that packages come down in the event of shipment from a factory. Such an occurrence is apt to damage the gears and other drive members built in the revolver. To form an image, the revolver has to be moved to and held at the developing position. Usually, the revolver is rotated to and held at a reference position or home position. However, since the revolver behaves relative to the casing when removed from the apparatus body, as stated above, the position of the revolver relative to the axis of rotation becomes arbitrary. As a result, when the revolver is mounted to the apparatus body afterwards, it cannot be rotated to the reference position without wasting time and cost. Moreover, the time and distance necessary for the revolver to return to the reference position increase, aggravating the erroneous detection of the reference position.
(6) Assume that, to apply a preselected bias voltage to each developing unit, the developer carrier of each developing unit, for example, has the shaft thereof extended to the outside of the casing and connected to an electrode. Then, the portion connecting the shaft and electrode is located outside of the casing and, therefore, apt to deform due to external forces. The deformation, as well as the deposition of impurities on such a portion, is apt to make the contact defective.
On the other hand, none of the previously mentioned Laid-Open Publication Nos. 60-162271 and 60-238872, for example, describes or even suggests a method of mounting the casing or cover to the revolver, a method of holding the casing in the apparatus body while preventing it from rotating, a method of handling the casing and revolver when the revolver is removed from the apparatus body, etc.